渤海湾西北湾顶泥沙运动特征及模拟

Characteristics and simulation of sediment concentration in the northwestern head of Bohai Bay

渤海湾西北湾顶处于粉沙质到淤泥质海岸过渡地带,泥沙运动复杂.根据三角架定点观测和水文测验资料,分析了不同水文条件下的含沙量变化特征,探讨了风浪掀沙模拟的关键处理方法.研究表明,研究海域泥沙运动与风浪关系密切,在小风天气下含沙量很小,六七级风以上天气条件下,且持续作用时,含沙量持续增大,较小风天气增幅达3~6倍左右;含沙量明显变化的临界波高约为0.5m.从研究海域观测的底部边界层水流结构、剪切力等角度进行了波浪掀沙的水动力机制分析.基于泥沙扩散方程,提出了风浪条件下泥沙模拟的处理方法,包括含沙量层化效应、制约沉速、动床阻力、参考浓度等.与实测资料比较表明,提出的计算方法基本可模拟风浪过程引起的含沙量变化.该研究不仅为类似海域提供基本数据,而且给出了模拟计算关键问题的处理方法,具有重要的理论和实践意义.

The evolution and utilization of estuarine and coastal regions are largely restricted by sediment issues. Waves and currents are the main dynamic driving forces of sediment transport in estuary and coastal areas. The study of wave-current movement and sediment transport is of great significance in both academic research and engineering practice, which has received much attention from many scholars and engineers. The northwestern head of Bohai Bay belongs to a transitional area from silty coast to muddy coast, where the sediment movement is complex. Firstly, based on the field observation data measured with a tripod system and the hydrological data in the northwestern head of Bohai Bay, the characteristics of tidal currents, waves, and suspended sediment concentration(SSC) changes under different hydrological conditions were analyzed. Sediment density distribution was detected by a γ-ray densitometer. Results show that the SSC is mainly influenced by wave-induced sediment suspension: Under light wind conditions the SSC was very low, with the peak value generally less than 0.1 kg/m^3;the SSC increased continuously under the gales over 6-7 in Beaufort scale, with sustained wind action. The measured peak SSC at 0.4 m above the seabed was 0.15-0.32 kg/m^3, with the average value of0.08-0.18 kg/m^3, which is about 3-6 times the value under light wind conditions. The density at the bed bottom was about1.4-1.6 g/cm^3, and decreased to 1.02-1.03 g/cm3 at 0.1-0.2 m above the bed;there was rarely little fluid mud in the study area.Secondly, the mechanism of wave-induced SSC was studied from the view of flow structure, shear stress in the bottom boundary layer. The friction velocity was calculated by fitting the datasets of the measured logarithmic velocity profile.Comparison shows that the wave shear stresses(calculated from the bottom velocity profile) is higher than the current shear stress(calculated from the upper velocity profile). Following the Reynolds’ decomposition method, expressions of waveinduced and current-induced shear stress were derived by splitting the variables into a fluctuating component, an averaged component and an oscillatory component. Compared with uniform flow, an additional item of wave streaming leads to a higher wave-induced shear stress.Finally, by analytically solving the sediment diffusion equation, a numerical procedure was proposed for sediment concentration under combined action of waves and currents. Some key approaches for fine sediment simulation were presented, including stratification effects, hindered settling, moving bed roughness, reference concentration and critical shear stress. Under combined wave-current conditions, the combined sediment diffusivity is given by the square sum of the wave-related and current-related diffusivities, and a toe-type distribution of wave-related eddy viscosity was employed.The stratification effects are considered by introducing the turbulence damping coefficient. The approaches of hindered settling for silt were employed. The enhanced roughness due to mobile bed effects was included. Comparison shows that,these approaches can properly simulate the measured wave-induced sediment concentration. This study provides basic data for coastal protection and utilization as well as for scientific study of silty-muddy coast, which is of great significance in practice and theory.